When a person breathes primarily through the nose (i.e., nasal breathing), one of the nostrils may be more dominant than the other nostril, with most of the exhaled air flowing through it. The right and the left nostrils may switch roles as the dominant nostril several times a day, and sometimes the breathing is essentially the same through both nostrils. Brain activity, and in particular, which hemisphere is relatively more effective, is correlated with the dominant nostril. When the left nostril is dominant, the right hemisphere (“right-brain”) is typically more effective at performing certain activities associated with it, and when the right nostril is dominant, the left hemisphere (“left-brain”) is typically more effective at performing certain activities associated with it.
Since each side of the brain plays a different role in different types of activities, it has long been believed by yoga practitioners that it is better to conduct certain activities when a certain nostril is dominant. For example, exercising, eating, digesting, and taking care of the body (e.g., defecating) are better done when the right nostril is dominant. Various forms of mental activity, such as planning, memorizing, writing, thinking, etc. are better done when the left nostril is dominant.